Dr. Tabrez Ahmad

Assistant Professor

Department of Zoology BSNV PG College, Lucknow

SCOLIODON: HABIT, MORPHOLOGY, ANATOMY AND

PHYSIOLOGY

FOR- B.Sc. IIIrd

SEMESTER, PAPER I, UNIT I

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Scoliodon: Habit, Morphology, Anatomy and Physiology

The class Elasmobranchii embraces a large variety of cartilaginous fishes. To give a

general idea of the class, the common example of the group, the dogfish, is selected

as an introductory type.

There are several genera of dogfishes available in the different parts of the earth.

The typical Indian genus, Scoliodon is described below. The genus is represented by

about nine species, of which four are very common in the Indian seas.

The genus Scoliodon is distinguished from other dogfishes by having an elongated

snout, depressed head and a compressed body. The teeth are similar in both the jaws.

The caudal pit and the sub-caudal lobe are prominent and distinct. The four Indian

species of Scoliodon are: Scoliodon sorrakowah, S. dumerilii, S. palasorrah and S.

walbeehmi.

Systematic Position:

Phylum Chordara

Subphylum Vertebrata (= Craniata)

Series – Pisces

Class – Elasmobranchii

Subclass – Selachii

Superorder – Selachoidei (= Pleurotremata)

Order – Lamniformes

Family – Carcharhiniidae

Scientific Name (Scoliodon sorrakowah Bleeker, 1853) (= Scoliodon walbeehmi,

Bleeker, 1856)

Habit and Habitat of Scoliodon:

The shark is a marine, carnivorous and predaceous animal. It eats small pelagic

schooling and bottom living bony fishes, including anchovies, codlet (Bregmacero-

tidae), burrowing gobies (Tripauchenidae) and Bombay ducks (Harpadontidae) as

well as shrimps and cuttle fish.

Both sexes mature between 1-2 years old and the males reach largest size at the age

of about 5 years and females reach largest size at the age of 6 years.

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A common tropical shark of continental and insular shelves close, frequently in

rocky areas. The species is abundant in Indian and Pakistani waters.

Geographical Distribution of Scoliodon:

Scoliodon has a wide geographical distribution ranging from Zanzibar to Ceylon (Sri

Lanka), Ceylon to the Malay Archipelago of the Indian Ocean, Bay of Bengal,

Eastern Pacific (from Mexico to Panama), West Indies and eastern coasts of South

America. Fossils of Scoliodon have been discovered in the geological strata from the

lower Eocene to later periods.

External Structures of Scoliodon:

Scoliodon is an elongated spindle-shaped animal. It has a laterally compressed body.

A fully-developed specimen of the genus attains a length of about 60 cm. The body

is divisible into head, trunk and tail. The head is dorsoventrally flattened and

terminates anteriorly into a dorsoventrally compressed snout. The dorsal side of

Scoliodon is dark-grey while the underside is pale white.

The trunk is more or less oval in transverse section. It attains maximum thickness in

the middle region and the body gradually tapers posteriorly into a long tail (Fig.

6.2A). The tail is also oval in cross-section (Fig. 6.2B) and bears a heterocercal type

of caudal fin, i.e., the posterior end of the vertebral column is bent upwards and lies

in the dorsal or epichordal lobe.

The mouth is a very wide crescentic aperture lying on the ventral side of the head

near its anterior end. It is bounded by upper and lower jaws, each is beset with one

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or two rows of sharply pointed and backwardly directed teeth (Fig. 6.2C) to catch

the slippery prey.

The teeth are replaced if these are broken. The teeth of Scliodon are modified scales.

The scales cover its body and extend inside the jaws to serve as teeth (Fig. 6.2D).

The transition of the placoid scales into teeth is amply recorded in the jaw regions.

Two prominent circular eyes are present. Each eye is provided with movable upper

and lower eyelids. The third eyelid or nictitating membrane can cover the whole eye

in emergency. The pupil is a vertical slit-like aperture (Fig. 6.2E).

The nostrils are placed one at each angle of the mouth. These are exclusively

olfactory in function and have no connection with the mouth cavity. Each nostril is

partly covered by a small fold of skin. Posterior to the eyes there are five vertical

slits on each side. They are called gill or branchial slits. The branchial slits lead into

the gill-pouches which in turn open into the pharyngeal cavity.

The cloaca opens to the exterior by a cloacal aperture which is located in between

the two pelvic fins. The cloacal aperture is an elongated opening. The cloaca is a

common chamber, into which anus, urinary and genital apertures open. On each side

of the cloaca lies the abdominal pores.

The abdominal pores are paired structures and situated on elevated papillae to

communicate the coelom to the outside. A faint lateral line is present. Beneath this

line a canal is present. The canal opens to the exterior by minute pores at intervals.

Many pores, called ampullary pores, are also present on the head.

Fins:

As in other fishes, Scoliodon bears unpaired and paired fins which are actually flap-

like integumentary extensions of the body. These are flexible and are stiffened by

cartilaginous rods or horny fin-rays. All the fins are directed backwards which is of

positive advantage in swift forward movement in water.

Median unpaired fins:

The fins under this category include two dorsals, one caudal and one ventral fin. The

dorsal fins are triangular in outline. The anterior dorsal is larger and situated at about

the middle of the body. The posterior dorsal is comparatively small and occupies a

median position between the first dorsal and the tip of tail.

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The caudal fin has one ill- developed ventral lobe (hypochordal) which is divided

into two parts. Two shallow depressions, called caudal pits, are regarded as the

diagnostic features of the genus. These are present at the root of the tail, one at the

dorsal surface and another on the ventral. The median ventral fin is located in the

mid-ventral line and just anterior to the caudal fin.

Lateral paired fins:

Two pectoral and two pelvic fins constitute the lateral paired fins. The pectoral fins

are large and are situated posterior to the gill-clefts. The pelvic fins are much

smaller. In females, these are simple but in males each of them is connected with a

copulatory organ called myxipterygium or clasper. Clasper is rod-like in appearance

having a dorsal groove leading to a siphon at its base.

Skin of Scoliodon:

The integument is composed of an outer epidermis and an inner dermis. The epider-

mis is composed of epithelial cells intermixed with numerous unicellular mucous

glands. In the young stage the epithelial cells are ciliated. But in an adult the cilia are

lost.

The dermis is composed of three layers:

(i) Stratum laxum,

(ii) Stratum compactum and

(iii) Subcutaneous layer.

The stratum laxum is the outer layer and lacks fibres. The median layer is the

stratum compactum which is fibrous in nature. The basal plate of the placoid scale is

tied to this layer by fibres. The subcutaneous layer is variable in thickness and

contains fine fibres arranged in a reticular fashion.

Muscular System of Scoliodon:

Between the endoskeleton and the skin there are closely packed muscles. The

muscles are highly developed in the trunk and tail regions and exhibit distinct

segmental arrangement. There are numerous myotomes which are separated from

one another by partitions made up of tough connective tissue called the

myocommata.

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The muscle fibres run parallel to the length of the body in each myotome. In the

head region, the muscles do not exhibit any sign of segmentation and have become

specialised to control the movement of the jaws, pharynx and eyes.

In the trunk region, the muscles are greatly thickened on the dorsal side on each side

of the vertebral column, whereas in the tail region the muscles are equally developed

round the vertebral column (see Fig. 6.2B).

Skeletal Structures of Scoliodon:

Scoliodon possesses well-developed exoskeletal and endo-skeletal structures. The

exoskeleton includes predominantly the scales which are present all over the body.

The endoskeleton embraces the axial and appendicular skeleton.

Exoskeleton:

The entire surface of the body is covered by oblique rows of placoid scales or

odontoids (Fig. 6.3). A typical placoid scale has a basal plate made up of calcified

tissue which remains embedded in the skin and a backwardly directed spine

projecting out of skin.

The basal plate is held in the dermis by Sharpey’s and other fibres (Fig. 6.3E). A

perforation is present at the base of the spine which communicates the pulp cavity of

the basal plate with the pulp cavity in the spine.

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The spine is composed of dentine coated externally with enamel. The nature of

enamel is controversial. It is called the fibro dentine, because it is formed as the

calcification of the fibrous material between the dentine and the enamel organ.

The scales covering the body extend inside the jaws, where they act as teeth. Placoid

scale has dual source of origin and develops partly from epidermis and partly from

dermis. The basal plate as well as the dentine of the spine is the derivatives of the

mesoderm. The enamel is derived from the ectodermal enamel organ.

Endoskeleton:

The endoskeleton is composed exclusively of cartilage which may be hardened by

calcification at places.

It can be described under two heads:

(a) The axial skeleton comprising of the skull and the vertebral column and

(b) The appendicular skeleton consisting of the pectoral and pelvic girdles and the

skeleton of the fins.

Axial skeleton:

The skull is a simple cartilaginous casket. It consists of the cranium, four sense

capsules enclosing the auditory and olfactory organs and the visceral skeleton which

form the jaws and support the pharynx with gills.

These three components of the skull remain intimately fused with each other (Fig.

6.4A, B).

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The cranium has four distinct regions:

(a) The occipital region forms the posterior portion of the skull and contains a large

foramen magnum through which the spinal cord passes down. An occipital condyle

is present on either side of the foramen magnum. Above this foramen there is a

median ridge, called the occipital crest.

(b) The auditory region is made of auditory capsules which remain firmly united

with the cranium in an adult. An oval depression between the two capsules is called

the parietal fossa. The roof of the cranium has two large fontanelles on the dorsal

side.

(c) The middle region of the skull is composed of orbit. The margin between the

roof of the cranium and the orbit is marked by the supraorbital ridge. A slender

cartilage, called pre- orbital process, partly encircles the orbit. Similarly a postorbital

process emerges from the side forward along the upper margin of the orbit.

(d) The anterior or the ethmoidal region of the cranium is composed of olfactory

capsules and rostrum. The two olfactory capsules are separated by internasal septum.

The visceral skeleton consists of seven half-hooped cartilaginous structures that

encircle the buccal cavity and the pharynx. The first pair of the visceral arches give

rise to the jaws, the second pair to the hyoid arch and the rest of them support the

gills.

The first visceral arch is the mandibular arch. Each half of this arch divides into two

parts, the upper part is called palatopterygoquadrate which forms the upper jaw and

the lower part is known as Meckel’s cartilage which forms the lower jaw.

The second arch is called hyoid arch which consists of three parts—a ventral

basihyal, a lateral ceratohyal and a dorsal hyomandibular. The suspension of the

jaws with the cranium is made through the hyomandibular and this type of

suspensorium is called hyostylic. The rest of the visceral arches are known as

branchial arches which support the pharynx and the gills.

Vertebral column:

The vertebral column is composed of a chain of cartilaginous vertebrae. The

vertebrae develop around the notochord which is persistent (Fig. 6.4C). The ver-

tebrae differ slightly along the length. A trunk vertebra is taken as the typical one.

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Structure of a trunk vertebra:

A trunk vertebra has a centrum that encloses the notochord. Above the centrum there

is a neural canal through which the spinal cord passes down. The neural canal is

enclosed by neural arch which contains a short blunt neural spine. There is a pair of

transverse processes which project from the centrum ventrolaterally. The centrum is

amphicoelous (i.e., the centrum exhibits concavities on both the ends).

The notochord is very narrow within centrum but becomes very much dilated in the

intervertebral spaces (see Fig. 6.4C). The centra are reinforced by calcified

fibrocartilage which forms four wedges and traverse the body of the centrum as a

cross. Such centra are generally called asterospondylous types.

The posterior vertebrae contain haemal arch which is present on the ventral side of

the centrum. The haemal arch encloses the haemal canal enclosing the caudal artery

and vein. The haemal arch gives a haemal spine to support the ventral lobe of the

caudal fin. The posterior end of the vertebral column is bent dorsally.

Appendicular skeleton:

The supporting skeleton of the fins (both unpaired and paired ones) constitutes the

appendicular skeleton. The two dorsal fins and the ventral fin are provided with

series of cartilaginous rod-like structures, called the pterygiophores or somactidia.

The distal ends of the somactidia bear double series of the ceratotrichia or horny fin-

rays. Somactidia are absent in the caudal fin. Caudal fin is supported by the

extensions of the neural and haemal spines.

Pectoral girdle and fin:

The pectoral girdle is situated posterior to the last branchial arch and consists of two

semicircular cartilages united with one another along the mid-ventral line. The

dorsal portion of each half is composed of a thick and rod-like scapula and the

ventral portion is made up of a thin and flattened corcacoid (Fig. 6.5A).

At the junction of the coracoid and the scapula there are three facets for the

articulation of the three basal cartilages of the pectoral fin called the propterygium,

mesopterygium and metapterygium. The basal cartilages bear many radial cartilages

(radials) supporting the pectoral fin.

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Pelvic girdle and fin:

The pelvic girdle comprises of a flattened cartilaginous rod situated along the

transverse plane in front of the cloaca. A curved basal cartilage, called

basipterygium, supports the radials of the pelvic fin (Fig. 6.5B). The basipterygium

is attached anteriorly to the pelvic girdle.

The radials at the distal ends contain small cartilages bearing the ceratotrichia. In

males each clasper is a tubular cartilage and grooved dorsally. Distally the groove

terminates into a sharp style which is enclosed by two sheathing plates. At the upper

end of the style lies a small accessory cartilage.

Locomotion in Scoliodon:

The movement of Scoliodon is caused by the activities of the myotomal longitudinal

muscle fibres and is also aided by movement of the fins. In the phylogenetic history

of the fishes, the fins were primarily employed to raise the body off the bottom, but

these become secondarily used in swimming by producing undulatory movements.

The longitudinal muscle fibres composing the myotomes play the important role in

swimming. The myotomes are placed on either side of the incompressible vertebral

column which acts as a lever upon which the myotomes work.

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The contractility of the myotomes thus causes the bending of the body. During for-

ward progression the contraction of the myotomes occurs along the anteroposterior

direction in such a way that the waves of curvature pass down each side of the body

alternately from the head to the tail.

Such contraction is called the metachronal contraction. It has been calculated that

about 54 waves are produced per minute during steady swimming.

The controlling factors governing such contraction of muscles are not fully known.

In Scoliodon, the transaction of the spinal cord behind the medulla oblongata is

capable of producing swimming movements for several days. This phenomenon

suggests that rhythm of contraction is largely governed by the spinal cord, but the

influence of brain on the whole process is yet unsolved.

The body and fins are specially modified to maintain an equilibrium in water during

swimming. The dorsal fin is well-developed and helps to restore stability and helps

in the restoration of equilibrium, if there is any deviation along the vertical axis of

the body. The pectoral fins play a very important role in turning of the fish by the

unilateral breaking with the pectoral fins.

The pectoral fins also help to maintain stability in the vertical plane. The movable

pectoral fins lift the head upwards and this is compensated by the heterocercal tail.

The hypochordal lobe is flexible and the epichordal part is rigid. The flexibility of

the hypochordal lobe gives a vertical lift of the tail. The pelvic fins have no utility in

locomotion and usually help in reproductive functions, specially in males.

Coelom of Scoliodon:

In Scoliodon, the coelom is spacious and is divided into a smallest pericardial cavity

and an extensive abdominal cavity. These two cavities are separated by the septum

transversum and communicate with one another through pericardioperitoneal canal

situated in the septum.

The abdominal cavity contains the viscera (Fig. 6.6) and opens to the exterior

through a pair of abdominal pores. The pericardial coelom houses the heart.

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Digestive System of Scoliodon:

The digestive system consists of the alimentary canal and the digestive glands. The

alimentary canal starts with the mouth and terminates in the anus. The mouth leads

into a spacious buccal cavity which is lined with mucous membrane.

The floor of the buccal cavity becomes folded to form a non-muscular and non-

granular ‘tongue’. The mucous membrane is very thick and rough due to the

presence of dermal denticles or teeth. The teeth are very sharp and are obliquely

placed (see Fig. 6.2C). The teeth are homodont (i.e., the teeth are similar in shape)

and lyodont (possesses several sets of teeth functioning in succession).

The buccal cavity leads into pharynx. On either side of the pharynx there lie the

internal openings of the spiracles and five branchial clefts. The mucous membrane

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of the pharyngeal wall contains numerous dermal denticles. The pharynx leads into a

narrow oesophagus. The inner mucous membrane of the pharynx is raised to form

longitudinal folds.

The oesophagus dilates posteriorly to form a large stomach. The stomach is highly

muscular and is bent on itself to form a J-shaped configuration. The long limb of the

stomach is continuous with the oesophagus and the shorter one passes into the

intestine. The entrance of the oesophagus into the stomach is provided with a

crescentic fold which serves as the valve.

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The long anterior limb is called the cardiac stomach and the short posterior limb is

designated as the pyloric stomach. A small outgrowth often called ‘blind sac’ is

present at the junction of the cardiac and pyloric limbs.

The inner lining of the cardiac stomach is folded longitudinally like that of

oesophagus (Fig. 6.7A). The internal lining of the pyloric stomach is mostly smooth

though slight foldings are observed at the distal end.

The pyloric valve, at the end of the pylorus, guards the entrance of it into a thick-

walled small chamber called the bursa entiana. The bursa entiana is immediately

followed by wide tubular intestine which becomes narrowed posteriorly as the

rectum. The rectum opens into the cloaca. A tubular caecal or rectal or digit form

gland opens into the rectum.

The inner surface of the intestine becomes folded to form an anticlockwise spiral of

approximately two and a half turns. This is called the scroll valve (Fig. 6.7B) which

increases the absorptive surface of the intestine and also checks the rapid flow of

digested food through the intestine.

The major digestive gland is the liver which is a massive yellowish gland and con-

sists of two lobes. The lobes are united anteriorly. A thin-walled V-shaped gall-

bladder is present in the anterior part of the right lobe of liver. The bile duct receives

a few smaller ducts from the two lobes of the liver and opens into the anterior end of

intestine near the commencement of the scroll valve.

The pancreas is a pale compact irregular body and consists of a dorsal lobe situated

parallel to the posterior part of cardiac stomach and a ventral lobe which remains

closely attached to the pyloric stomach. The pancreatic juice is poured into the

intestine by pancreatic duct situated opposite to the aperture of the bile duct.

The functional significance of rectal gland is not properly known. The rectal gland

has a central cavity lined with cuboidal cells. It is highly vascular and composed of

lymphoid tissue. It discharges a fluid into the lumen of the intestine but its actual

role is not known.

The buccal cavity possesses no such glands that can be compared with the salivary

glands of higher vertebrates. The spleen is located dorsal to the distal end of the

body of the stomach. The spleen is functionally associated with the circulatory

system, but remains morphologically connected with the alimentary canal.

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Respiratory System of Scoliodon:

The respiratory organs are the gills which are borne by the gill-pouches. The

structure of gill-pouches differs in different dogfishes. Fig. 6.8B shows the

organisation of gill-pouch of Brachaelurus, a related genus of Scoliodon. There are

five pairs of gill-pouches, each of which communicates with the pharyngeal cavity

by a large internal branchial aperture and opens to the outside by exterior gill-slits

(Fig. 6.8A).

The mucous membrane lining of the gill- pouches gives a series of horizontal

branchial lamellae. The branchial lamellae are highly vascularized structures.

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Each gill-pouch has an anterior set of branchial lamellae and a posterior set of

branchial lamellae. The gill- pouches are separated by inter-branchial septum which

projects beyond the branchial lamellae (Fig. 6.8C).

The pharyngeal end of each inter-branchial septum is supported by a visceral arch.

Each arch supports the anterior set of lamellae of one gill-pouch behind and the

posterior set of lamellae of the next gill- pouch. The first gill-pouch lies between the

hyoid and the first branchial arches and the last one is present between the fourth

and fifth branchial arches.

There are two types of gills:

(i) Holobranch or complete gill when a branchial arch bears two sets of gill lamellae

and

(ii) Demi branch or hemi branch or half gill when single set of gill lamellae is

present. The hyoid arch supports only a demi branch and the first four branchial

arches support holobranchs. The last branchial arch is gill-less.

Mechanism of respiration:

During respiration the floor of the buccal cavity is lowered and the mouth is opened.

Then the water rushes in to fill the greatly expanded buccal cavity. The mouth is

now closed and the pharynx contracts.

The water then enters the gill-pouches and goes out after gaseous exchange through

gill-slits. The spiracles are occasionally used as accessory pathways for the entry of

water for respiration instead of the mouth when it is otherwise occupied.

Circulatory System of Scoliodon:

The circulatory system consists of:

(a) The circulatory fluid, called blood,

(b) The heart,

(c) The arteries and

(d) The veins.

Blood:

The blood consists of a colourless plasma and corpuscles are suspended in the

plasma. Two kinds of corpuscles are encountered; the RBC (or erythrocytes) and the

WBC (or leucocytes). The erythrocytes are oval bodies containing a nucleus. The

haemoglobin is present in the erythrocytes. The leucocytes are amoeboid in

structure.

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Heart:

The heart is a bent muscular tube and consists of the receiving parts, comprising of a

sinus venosus and a dorsally placed auricle, and the forwarding parts, consisting of a

ventricle and a conus arteriosus (Fig. 6.9A). The heart is situated on the ventral side

of the body between two series of gill-pouches.

Receiving parts of the heart:

The sinus venosus is a thin-walled tubular chamber. The sinus venosus is highly

contractile and the beating of the heart originates from this part of the heart. Two

great veins, the ductus Cuveiri, open into the sinus venosus, one on each lateral side.

Two hepatic sinuses enter the sinus venosus posteriorly. The sinus venosus opens

into the auricle by sinuauricular aperture which is guarded by a pair of valves. The

auricle is a large, triangular and thin-walled chamber situated dorsal to the ventricle

but in front of the sinus venosus.

The auricle communicates with the ventricle through a slit-like auriculoventricular

aperture guarded by two lipped valves. The receiving chambers, (sinus venosus and

auricle) receive the venous blood from all parts of the body.

Forwarding parts of the heart:

The ventricle has a very thick muscular wall, the inner surface gives many muscular

strands, thus giving it a spongy texture (Fig. 6.9B). It is an oval chamber and

constitutes the most prominent part of the heart. The conus arteriosus is a stout

median muscular tube arising from the ventricle.

The lumen of the conus arteriosus is provided with two transverse rows of semi-

lunar valves. To keep the valves in position the free ends of the valves are attached

to the ventricular wall by fine tendinous threads, called chordae tendinae. The conus

arteriosus is continued forward as the ventral aorta.

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The function of the heart is to receive the deoxygenated blood from all parts of the

body and to pump it for aeration to the gills. Such a type of the heart is designated as

the venous or branchial heart, because only the deoxygenated blood circulates

through its different parts.

Arterial System of Scoliodon:

The arterial system of Scoliodon is divided into two distinct categories of arteries.

These are:

(a) The afferent branchial arteries arising from the ventral aorta which bring the

deoxygenated blood to gills for oxygenation and

(b) The efferent branchial arteries which originate from gills and convey the

oxygenated blood to the different parts of the body (Fig. 6.10).

Afferent branchial arteries:

The ventral aorta is situated on the ventral surface of the pharynx and extends up to

the posterior border or the hyoid arch. The ventral aorta divides into two branches

called innominate arteries, which again bifurcates into the first and second afferent

branchial arteries.

The third, fourth and fifth afferent arteries arise from the ventral aorta. Each afferent

branchial artery arises from the ventral aorta by independent opening except the

anterior most pairs which arise by a common opening (Fig. 6.10).

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Efferent branchial arteries:

The afferent branchial arteries break up into capillaries in the gills. From the gills the

blood is collected by efferent branchial arteries (Fig. 6.10). There are nine pairs of

efferent branchial arteries and these are equally distributed on each side.

The first eight arteries form a series of four complete loops around the first four gill-

slits and the ninth efferent branchial artery collects blood from the demi branch of

the fifth gill- pouch and from where blood is poured into the fourth loop.

In addition to short longitudinal connectives connecting the four loops, these are

further connected with each other by a network of longitudinal commissural vessels

called the lateral hypobranchial chain.

From each efferent branchial loop arises an epibranchial artery. The four pairs of

epi- branchials join in the mid-dorsal line to form the dorsal aorta. The ninth efferent

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branchial artery has no epibranchial branch but joins with the eighth efferent

branchial artery.

Anterior arteries:

The head region gets the blood supply from the first efferent branchial artery and

partly from the proximal end of the dorsal aorta.

Arteries from the first efferent branchial (hyoidean efferent) are:

(a) The external carotid,

(b) The afferent spiracular and

(c) The hyodean epibranchial which in turn receives a branch from dorsal aorta.

The external carotid artery originates from the first collector loop and divides into a

ventral mandibular artery giving branches to the muscles of the lower jaw and a

superficial hyoid artery which supplies the second ventral constrictor muscle, the

skin and the subcutaneous tissue beneath the hyoid arch.

The afferent spiracular artery after originating from the middle of the hyoidean

efferent, proceeds forward as the spiracular epibranchial artery and enters the cranial

cavity. Just before its entry into the cranial cavity it sends a great ophthalmic artery

to the eye ball. Immediately after the entry to the cranium it joins with a branch from

the internal carotid to form the cerebral artery.

The cerebral artery immediately divides into an anterior and a posterior cerebral

artery which supply the brain. The hyoidean epibranchial artery runs forwards and

inwards to the posterior border of the orbit and gets an anterior branch from the

dorsal aorta.

It divides immediately into:

(a) the stapedial artery which gives off the inferior orbital artery and runs forward as

the superior orbital artery supplying the six eye muscles and the superficial tissue

above the auditory capsule. The superior orbital artery gives a large buccal artery

which runs as the maxillonasal artery. The maxillonasal gives several arteries to the

muscles of the upper jaw, the olfactory sac and the rostrum,

(b) the internal carotid artery passes inward and enters the cranium where it

bifurcates into two branches. One of the branches unites with its fellow from the

opposite side and other branch unites with the stapedial.

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Dorsal aorta and its branches:

The dorsal aorta is formed by the union of epibranchial arteries. It runs posteriorly

and is situated ventral to the vertebral column. It is continued up to the tip of the tail

as the caudal artery.

Along the anteroposterior direction the following arteries have their origin

from the dorsal aorta:

(a) Several buccal and vertebral arteries are given off anteriorly.

(b) A pair of small subclavian arteries arise from near the origin of the fourth

epibranchial arteries.

The subclavian artery gets the epicoracoid artery on its way and divides into:

(i) A branchial artery to the pectoral girdle and pectoral fin,

(ii) An anterolateral artery to the body musculature and

(iii) A dorsolateral artery to the dorsal musculature.

(c) A large coeliacomesenteric artery arises just behind the origin of fourth

epibranchial artery. It divides into a smaller coeliac artery and a larger anterior

mesenteric artery.

(d) A lienogastric artery originates posterior to the coeliacomesenteric artery and

gives off

(i) An ovarian (in females) or spermatic artery (in males) to gonad,

(ii) A posterior intestinal artery to the posterior part of the intestine,

(iii) A posterior gastric to the posterior part of the cardiac stomach and

(iv) A splenic artery to the spleen.

(e) Series of paired parietal arteries emerge out behind the subclavian artery. Each

parietal gives a dorsal parietal artery and a ventral parietal artery. The dorsal parietal

artery supplies the dorsolateral musculature, the vertebral column, the spinal cord

and the dorsal fin. The ventral parietal artery supplies the ventral muscles and the

peritoneum. The ventral parietal gives renal branches to the kidneys.

(f) A pair of iliac arteries extend to the pelvic fin as femoral arteries.

Hypobranchial chain:

A lateral hypobranchial chain is formed by a network of slender arteries arising from

the ventral ends of the loop of the efferent branchial arteries. Four commissural

vessels arise from the lateral hypobranchial chain which on the ventral wall of the

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ventral aorta unites to form a pair of median hypo-branchials which communicate

with one another by transverse vessels.

Posteriorly the median hypo-branchials unite to form a median coracoid artery

which gives rise to the coronary artery and a pericardial artery. The pericardial

artery gives off the common epicoracoid artery which in turn divides into left and

right epicoracoid arteries each joining one subclavian artery.

Venous System of Scoliodon:

The deoxygenated blood from the different parts of the body is returned to the heart

by veins which form irregular blood sinuses throughout their courses (Fig. 6.11).

The existence of extensive blood sinuses is a characteristic feature of the venous

system of Scoliodon.

The venous system is extremely complicated and is described under the

following heads:

(A) Cardinal system:

The blood from the anterior region of the body is returned to the heart by paired

jugular and anterior cardinal sinuses. The blood from the posterior region is

collected by a pair of posterior cardinal sinuses. The anterior and posterior cardinals

unite on each side to form a transverse sinus called ductus Cuvieri.

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(i) Anterior cardinal system:

This system of veins returns blood from the head region and consist of a pair of

internal jugular veins. Each internal jugular vein is composed of the olfactory sinus,

the orbital sinus, the postorbital sinus and the anterior cardinal sinus. The blood

from the rostral region is drained by the anterior facial vein to the olfactory sinus

and from there to the orbital sinus.

The orbital sinus opens into the anterior cardinal sinus through the postorbital sinus.

The anterior cardinal sinus enters the ductus Cuvieri. The anterior cardinal sinus

receives the hyoidean sinus and five dorsal nutrient branchial sinuses from the gills.

(ii) Posterior cardinal system:

The caudal vein collects blood from the tail region and proceeds forwards through

the haemal canal. In the abdominal cavity, the caudal vein divides into left and right

renal portal veins which break up into sinusoid capillaries in the substance of the

kidneys.

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Throughout its length, the renal portal vein receives small parietal veins. The renal

veins collect blood from the kidneys and unite to form the posterior cardinal sinuses.

Two posterior cardinal sinuses open into the ductus Cuvieri.

(B) Hepatic portal system:

A large number of small veins carrying blood from the alimentary canal and its

associated glands unite to form the hepatic portal vein. The hepatic portal vein

receives the lienogastric vein and anterior and posterior gastric veins.

Actually the hepatic portal vein is formed by the confluence of the anterior and

posterior intestinal veins. The hepatic portal vein breaks up into capillaries in the

liver. From the liver blood is collected by another set of capillaries which unite to

form two large hepatic sinuses opening into the sinus venosus.

(C) Cutaneous system:

This system consists of a dorsal, a ventral and two paired lateral cutaneous veins.

The inferior lateral cutaneous vein joins with the lateral cutaneous vein near the

anterior end of the pectoral fin. Each lateral cutaneous vein ultimately opens into the

brachial vein.

(D) Ventral system:

This system comprises of two sets of veins—the anterior ventral veins pouring blood

to the ductus Cuvieri through inferior jugular sinuses and the posterior veins which

discharge through the subclavian vein.

Each inferior jugular sinus is formed by the union of the sub-mental sinus from the

lower jaw, the hyoidean sinus and the ventral nutrients from the gills. Each inferior

jugular vein opens to the ductus Cuvieri. The subclavian vein also opens to each side

of the ductus Cuvieri.

Two large lateral abdominal veins are formed by a small caudal vein and two iliac

veins. The lateral abdominal veins are connected posteriorly by a commissural vein.

Anteriorly the lateral abdominal vein joins the brachial vein to form the subclavian

vein which in turn opens to the ductus Cuvieri.

Nervous System of Scoliodon:

The nervous system of Scoliodon includes:

(i) The central nervous system,

(ii) The peripheral nervous system and

(iii) The autonomous nervous system.

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Central nervous system:

The central nervous system consists of brain and the spinal cord.

Brain:

The brain is highly organised and shows many advancements over that of the

agnathans.

The brain is divided into three primary parts:

(a) The forebrain or prosencephalon,

(b) The midbrain or mesencephalon and

(c) The hindbrain or rhombencephalon.

The forebrain consists of a massive undivided cerebral hemisphere. The cerebral

hemisphere is relatively larger than that of other fishes. From the anterior end of

cerebral hemisphere arise two stout olfactory peducles, each terminates into a large

bilobed olfactory lobe (Fig. 6.12).

The olfactory lobes lie close to the olfactory capsules. Each olfactory nerve is

composed of many bundles of nerve fibres. The surface of the cerebrum is smooth

and the walls are thick.

A small opening called the neuropore is present on the mid-ventral surface of the

cerebrum. The posterior part of forebrain (diencephalon) is very short. The roof of

the diencephalon is thin, non-nervous and contains the anterior choroid plexus. The

lateral walls of the diencephalon form two thickened bodies, called thalami.

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A long and slender tube, the pineal organ or epiphysis cerebri projects from the roof

of the diencephalon up to the membrane covering the anterior frontanella. The floor

of the diencephalon (or hypothalamus) is well-formed. A hollow infundibulum is

given off from the floor of the diencephalon.

The infundibulum is dilated to form two oval thick-walled bodies, called lobi

inferiores whose distal ends are produced into two thin-walled glandular sacs, called

sacci vasculosi. The lobi inferiors are the centres for gustation and smell.

The sacci vasculosi is believed to be a centre for reception of the pressure of

cerebrospinal fluid and at the same time they produce cerebrospinal fluid. The

hypophysis is attached to the infundibulum. The optic chiasma lies in front of the

infundibulum. The optic chiasma is formed by the decussation of the nerve fibres of

two optic nerves (Fig. 6.12B).

The midbrain is large and consists of two round optic lobes. The optic lobes are

situated behind the diencephalon. The floor and the side walls are relatively thicker.

The midbrain is considered as the centre of co-ordination.

The hindbrain consists of a highly developed cerebellum and a medulla oblongata.

The dorsal surface of the cerebellum produces many irregular convolutions. The

cerebellum contains a small cavity. The cerebellum is also a centre of co-ordination.

The cerebellurp is divided into three lobes by two well-marked transverse furrows.

The medulla oblongata is triangular and the anterior end gives a pair of hollow

corpora restiformia with trace of convolutions in adults. The medulla controls res-

piration. Two corpora restiformia are connected by the transverse nerve band. The

roof of the medulla oblongata is non-nervous and bears the posterior choroid plexus.

The hind- brain controls swimming movements.

The ventricles of the brain are moderately developed (Fig. 6.12C). The cerebral

hemispheres contain narrow lateral ventricle. The third ventricle is extended forward

about half the length of the cerebral hemispheres. The floor of the fourth ventricle is

very much thickened. The fourth ventricle is large and extends dorsally into the

cerebellum and is continuous behind with the cavity of the spinal cord.

The iter (i.e., the communicating duct between the third and the fourth ventricles) is

wider. Although the cerebrum is undivided, there are two lateral ventricles which are

continued to the rhinocoels (cavity of the olfactory lobes).

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Spinal cord:

The spinal cord in Scoliodon shows definite advancement towards the plan of higher

vertebrates, The spinal cord is provided with pia mater only. The grey matter is

arranged into the dorsal and the ventral horns. The dorsal horns are united to form a

single broad region, as a result the grey matter assumes a shape of an inverted ‘T’.

Peripheral nervous system:

The peripheral nervous system includes the cranial nerves and spinal nerves.

Cranial nerves:

There are ten pairs of cranial nerves in all the fishes. An extra pair of anterior

terminal or pre-olfactory nerves is present in Scoliodon. The terminal nerves are des-

ignated as cranial nerve no. 0. The first pair of cranial nerves are the olfactory nerves

which originate from the olfactory lobes and innervate the olfactory sacs.

The terminal nerves are situated between the two olfactory lobes. These nerves

emerge from the telencephalon and bear a ganglion, called ganglion terminate, near

the origin. These nerves supply the nasal septum and the external nostril. The second

pair of cranial nerves are the optic nerves which, after the origin from the optic

thalami, form the optic chiasma and supply the eyes.

The third cranial nerve is called oculomotor nerve which originates from the ventral

surface of the mesencephalon and supplies the anterior, superior and inferior recti

and the inferior oblique muscles of each eye ball. The fourth cranial nerve is called

trochlear or pathetic nerve which arises from the dorsolateral surface of the midbrain

and supplies the superior oblique eye muscle.

The fifth cranial nerve is the trigeminal which has three branches:

(a) Ophthalmicus superficialis which supplies the skin of the snout;

(b) The maxillaris which is divided into maxillaris superior supplying nerves to the

skin of the upper jaw and maxillaris inferior innervating the posterior part of the

upper jaw;

(c) The mandibularis innervating the muscles of the lower jaw.

Another nerve called ophthalmicus profundus becomes secondarily associated with

the trigeminal to supply nerves to the eye ball and the dorsal surface of the snout

(Fig. 6.13). The sixth cranial nerve is the abducens which supplies the posterior

rectus muscle of the eye ball.

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The seventh cranial nerve is known as facial which divides into two branches:

(i) The ophthalmicus superficialis branch like that of the fifth cranial nerve and

(ii) A bundle of mixed nerves which subdivides into three routes:

(a) A ramus buccalis innervating the infraorbital canal of the snout,

(b) A ramus hyomandibularis supplying nerves to the lower jaw and throat and

(c) A ramus palatinus giving nerve supply to the roof of the buccal cavity and the

pharynx. The eighth cranial nerve is called auditory which gives the vestibular and

saccular branches to the internal ear.

The ninth cranial nerve is the glossopharyngeal which, in the region of the first gill-

cleft, divides into a small pretrematic nerve and a large posttraumatic nerve. These

nerves supply branches to the pharynx, pharyngeal muscles and the mucous

membrane surrounding the first gill-slit. The tenth cranial nerve is the vagus which

arises by multiple roots and gives off many branches.

The branches are:

(i) The brachial nerves supplying the gills,

(ii) The lateralis supplying the lateral line sense organs and gives numerous branches

along its course.

Terminal nerve (Cranial nerve ‘0’):

A new cranial nerve was first recorded in 1894 which connects the anterior end of

cerebral hemispheres. It was first identified in Protopterus and had since been

recorded in all gnathostomes excepting birds.

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To avoid confusion which would result if the long established nomenclature and

symbols of other cranial nerves were altered, this newly reported cranial nerve was

named terminal nerve or cranial nerve ‘0’. Some workers on this line are of the

opinion that nerve ‘0’ is a ganglionated remnant of the first branchial nerve. This

nerve is apparently unrelated to the olfactory nerve.

The terminal nerve leaves the anteroventral portion of the cerebral hemisphere and

passes into the mucous membrane of the olfactory organ. It is a sensory nerve and

possesses one or more ganglia. The terminal nerve is best developed in

elasmobranchs.

In amphibians, reptiles and mammals the terminal nerve is intimately associated

with the vomeronasal organ (Jacobson’s organ). The functional significance of the

terminal nerve is not yet fully ascertained. It has been suggested that it may repre-

sent the remnants of an anterior branchial nerve which has lost its significance in

course of evolution.

Spinal nerves:

The spinal nerves arise from the spinal cord. Each has one dorsal and one ventral

root. The dorsal root bears a ganglionic swelling. After emerging out through the

vertebral column, the dorsal and the ventral roots unite to form a common mixed

nerve.

Each spinal nerve gives three branches, such as:

(a) Ramus dorsalis,

(b) Ramus ventralis and

(c) Ramus communicans to join with the autonomous nervous system.

Autonomous nervous system:

This system is made up of a series of paired ganglia arranged irregularly on the

dorsal wall of the kidney and the posterior cardinal sinuses. The gastric ganglion is

the largest ganglion and sends nerves to the viscera. There is usually one ganglion in

each segment. In Scoliodon the successive ganglia are not in distinct continuous

chain.

Sense Organs of Scoliodon:

The nervous system is associated with highly developed sense organs, viz., eyes,

nose, ear and many others (Fig. 6.14).

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Eyes:

The eyes are built on the principle of a photographic camera. The eye ball is com-

posed of three layers: the sclera, choroid and retina. The sclera is cartilaginous. The

pupil is a vertical slit and it cannot be dilated or contracted. The retinal layer consists

of rods and cones.

A large number of guanine plates are present on the inner surface of the choroid

layer which is called tapetum lucidum. It acts as reflector. The crystalline lens is

spherical. The lens is kept in position by suspensory ligament which extends from

the margins of the lens to the ciliary processes.

The ciliary processes are longitudinal folds of the choroid layer. The eye is kept in

its position in the orbit by six extrinsic eye muscles. Besides the eye muscles there

are an optic nerve and a cartilaginous optic pedicle (Figs. 6.15 and 6.16).

These eye muscles are attached with the eye ball in two groups.

The first group include:

(i) Superior rectus,

(ii) Inferior rectus,

(iii) Anterior rectus and

(iv) Posterior rectus.

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The second group include:

(i) Superior oblique and

(ii) Inferior oblique.

The superior rectus muscle runs outwards and upwards and is inserted on the dorsal

side of the eye ball. The inferior rectus muscle extends outwards and downwards to

be inserted on the ventral side of the eye ball.

The anterior rectus muscle runs outwards and downwards and is attached to the

ventral surface of the eye ball. The posterior rectus muscle runs backwards and is

attached with the posterior surface of the eye ball.

The superior oblique muscle is attached on the dorsal surface of the orbit just in

front of the insertion region of the superior rectus muscle. The inferior oblique

muscle is attached to the ventral surface of the eye ball just in front of the inferior

rectus muscle.

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The eyes of Scoliodon are quite prominent and are proportionately larger in size.

The eyes are laterally placed and each eye has its own range of vision, i.e.,

Scoliodon has monocular vision. Scoliodon is possibly colour-blind (Fig. 6.16).

Olfactory organs:

There are two blind sac-like olfactory organs situated in front of the mouth. Each

olfactory sac is placed in a cartilaginous capsule and does not communicate with the

buccal cavity. The mucous membrane of the olfactory sac is thrown into two series

of folds called Schneiderian folds which are placed by the median raphe (see Fig.

6.14B).

The Schneiderian folds are composed of olfactory sense cells and supporting cells.

The olfactory sense organs are greatly developed in Scoliodon. Physiologically each

nasal opening is divided by three muscular nasal valves into a median ex-current

siphon and a lateral incurrent siphon.

Internal ear:

The internal ear or the membranous labyrinth of Scoliodon is a closed ectodermal

sac. It consists of three semicircular canals and a central body differentiated into an

anterior utriculus and a ventral sacculus (see Fig. 6.14A). The three semicircular

canals are arranged in three planes of the body and open into the central body by

both ends. But one end of each canal dilates to form ampulla.

The utriculus gives off an invagination called recessus utriculi beneath the ampullae

of the anterior vertical and horizontal semicircular canals. The sacculus gives a

posterior outgrowth called the lagena. The membranous labyrinth is placed in a

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cartilaginous auditory capsule. The inner cavity is filled with a fluid called the

endolymph into which many calcareous bodies (otoliths) are present.

The space between the membranous labyrinth and the auditory capsule is filled with

the perilymph. A long tube called the endo-lymphatic duct communicates the cavity

of sacculus to the exterior through a small opening.

The membranous labyrinth of Scoliodon performs three functions:

(a) It helps in orientation in relation to gravity,

(b) It accelerates in changing the direction during swimming and

(c) It helps in hearing.

The utriculus together with the semicircular canals is responsible for the orientation

and acceleration, while the sacculus is meant for hearing. The internal ears are the

statoacoustic organs in Scoliodon.

Neuromast organs:

These organs comprise of:

(i) The lateral line receptors and

(ii) The pit organs.

The lateral line sense organs lie inside the lateral line canal. These sense organs are

the epidermal derivatives and are called neuromasts which remain embedded in the

wall of the lateral line canal. This canal communicates to the exterior by minute

pores.

The pit organs occur on the lateral and dorsal sides of the head. They are actually

ectodermal pits connected by groups of sense-cells. The neuromast organs help to

orient the body in relation to currents and waves and are designated as

rheoreceptors.

Ampullae of Lorenzini:

Enumerable pores on the dorsal and the ventral sides of the head lead into a long

tube which terminates into radially arranged ampullary sacs (see Fig. 6.14C). The

ampullae lie in clusters and each of these consists of eight or nine chambers

arranged radially around a central core called centrum (see Fig. 6.14D).

Two types of cells are encountered in the ampullae—glandular cells and sensory

cells. The ampullae get the names according to their location. These sense organs are

the thermo receptors and are also the pressure receptors.

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Urinogenital System of Scoliodon:

The excretory organ consists of a pair of elongated kidneys. The functional adult

kidneys are called opisthonephros type according to Graham Kerr. The anterior

portion of the kidney is non-functional and the posterior portion becomes greatly

developed.

The kidneys are composed of coiled glandular uriniferous tubules or nephrons. Each

tubule consists of a double-walled cup (or Bowman’s capsule) enclosing glomerulus

and a much coiled renal tubule. A few renal tubules open into a common collecting

tubule. The kidney tubules have the peculiar property of reabsorbing urea.

The collecting tubules of the anterior non-renal portion of the kidney open to the

Wolffian duct and the posterior tubules open into the ureter which in turn opens into

the urinogenital sinus. The mesonephric duct divides into two.

The dorsal one is named as Wolffian duct and the ventral one is the Mullerian duct.

The Wolffian duct in males, becomes the vas deferens which is connected with the

vasa efferentia from the testis. The Mullerian duct becomes the oviduct in females.

The kidneys are extremely elongated structures extending from the root of the liver

to the cloacal region. The posterior portion is designated as the kidney proper and it

is greatly developed. The anterior portion becomes narrower and takes part in

conveying the genital products. This part is designated as the organ of Leydig or

epididymis.

Male reproductive system:

The testes are paired elongated organs (Fig. 6.17A). Each testis is attached to the

dorsal body wall by peritoneal membrane called mesorchium and posteriorly

attached by ordinary tissue with the caecal gland. The sperm cells escape by vasa

efferentia into the vas deferens which becomes extremely coiled in the anterior por-

tion of the kidney.

Posteriorly the vas deferens becomes very much dilated to form the seminal vesicle.

The seminal vesicles open into the urinogenital sinus which in turn opens into the

cloaca. The wall of the urinogenital sinus has become evaginated to form a sperm

sac. A pair of sacs designated as siphons are present. The siphons are located under

the skin on the ventral aspect of the body.

In posterior of the sacs these are continued as the siphon tubes which open to the

groove of the clasper of the respective sides (Fig. 6.17C). The siphons are not

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connected with the genital system. They contain sea-water and help in the expulsion

of the sperms through the clasper groove.

Female reproductive system:

In females there is no connection between the kidneys and the genital organs. The

kidneys are typical excepting that the ureters unite posteriorly and open by a single

urinary aperture into the urinary sinus. The ovaries are two in number and are kept

in position by peritoneal folds called mesovarium.

The shape, size and colour of the ovaries vary greatly according to the age of the

individual. A pair of epigonial organs is present between the ovary and the rectal

gland. The oviducts are very long tubes and remain united both posteriorly and

anteriorly (Fig. 6.17D). Posteriorly, two oviducts unite to form the vagina which

opens into the cloaca.

Anteriorly the oviducts converge and open into the coelomic cavity by a longitudinal

slit-like opening designated as oviducal funnel. At the anterior portion of each

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oviduct there is a dilated shell gland which has but little significance in Scoliodon.

As Scoliodon is viviparous the posterior portion of the oviduct becomes dilated to

form the uterus for the development of the young.

The number of embryos present inside one uterus varies greatly in different species

of Scoliodon. In S. sorrakowah about seven embryos have been observed. The

mucous membrane of the uterus becomes divided into a number of compartments

each of which houses an embryo. The number of compartments varies directly with

the number of embryos.

The eggs are large and heavily yolked. Each egg gets a coating of albumen during its

transit down the oviduct. The egg, particularly in oviparous forms, is enclosed by

horny shell which is oblong in shape. The angles of the shell are prolonged into four

coiled elongated filamentous processes (Fig. 6.17B). This condition is not observed

in Scoliodon but is observed in case of oviparous sharks.

Development of Scoliodon:

Scoliodon is viviparous and gives birth to living youngs. Fertilization is internal. It

is claimed that during copulation the claspers are introduced into the cloacal aperture

of the female for the transmission of spermatozoa. The egg of Scoliodon is strongly

telolecithal. The cleavage is restricted to a small germinal disc which floats on the

top of the yolk mass (Fig. 6.18A).

The cleavages are meroblastic and the first cleavage plane is simply a furrow in the

surface of the germinal disc. The second cleavage occurs at right angle to the first

one (Fig. 6.18B). After this stage, the cleavage plane is irregular. By this way of

cleavage, a blastodisc is separated from a layer of periblast cells. The periblast cells

form a syncitial layer over the yolk mass.

A blastocoel is formed between the blastodisc and the central region of the periblast.

With the expansion of the blastocoel the blastodisc becomes multi- layered. The

posterior region of the blastodisc grows faster than the other regions and is raised

from the yolk mass to become a double- layered germ ring. This germ ring is raised

posteriorly to form a blastopore (Fig. 6.18D).

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At this stage an upper layer (epiblast) and a lower hypoblast is separated. The

blastocoel lies between the ectoderm (epiblast) and the inner hypoblast

(mesendoderm). The process of gastrulation actually starts with the formation of the

dorsal lip of the blastopore. The endoderm is differentiated by cellular proliferation

from the underside of the hypoblast. The mesoderm produces a prechordal plate.

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38

As the embryo elongates along the anteroposterior axis, a head fold is marked off

from the blastodisc and it becomes raised up to form the neural folds. The body of

the embryo is constricted from the blastodisc and the tail fold is differentiated and

extended backward.

The blastopore is separated into two (embryonic and vitelline blastopores) by the

fusion of the edges of the yolk sac. The circulatory system appears in the mesoderm

in the form of blood islets which unite to form vascular networks. As the embryo

grows, the mouth is formed from the stomodeal invagination and the anus is formed

anew from the proctodeal invagination because the embryonic blastopore becomes

closed.

The developing embryo is provided with a tubular yolk stalk. This yolk stalk

connects the intestine of the embryo with the yolk sac. The yolk sac contains yolk

material which provides nutrition for the developing embryo. When the yolk is fully

exhausted the yolk sac becomes folded and becomes anchored with the uterine wall

of the mother as the yolk-sac placenta.

With the formation of placenta, the yolk stalk is lost and the blood vessels of the

yolk stalk form the placental cord. The placental cord connects the embryo with the

yolk-sac placenta (Fig. 6.18E). The placental cord develops many finger-like

processes called appendicula.

Each appendicularium is made up of a central core of loose connective tissue

surrounded by several layers of epithelial cells. The distribution of the appendicula

in the placental cord varies in different regions. The appendicularia help in the

absorption of nutrients secreted from the uterine wall of the mother.